This invention relates generally to a method for reinforcement of hollow thermoplastic storage vessels with one or more wraps of continuous fibers and more particularly to a means for improved bonding between the applied fibers and the outer vessel surface for storage vessels having relatively thin walls.
In a co-pending application Ser. No. 09/327,003 entitled “Reinforced Thermoplastic Pipe Coupling” and filed Jun. 7, 1999, now U.S. Pat. No. 6,164,702 in the names of David E. Hauber, Robert J. Langone and James A. Mondo which is also assigned to the present assignee, there is disclosed a continuous fiber reinforced thermoplastic pipe coupling having improved resistance to applied stress when used with pipe lengths being joined together. The fiber reinforcement is aligned during placement in a particular manner and placed at predetermined fiber angles dictated by mechanical forces being applied such as by internal fluid pressure in the coupled pipe lengths. Said already known method for construction of said reinforced thermoplastic pipe coupling includes a controlled directional orientation of the fiber component to enable the fiber placement to be fixed for maximum effectiveness in withstanding the particular stress being generated when the joined together pipe lengths are customarily used for the transfer of pressurized fluids. Since the fiber materials currently used in this manner are generally stronger than the polymer matrix compositions also being employed, the overall strength produced in the composite member depends largely upon the fiber placement direction for the particular end product. The fiber reinforced coupler is thereby only as strong as the spatial direction of the included fibers with respect to the direction of the internal stress when applied to said member. Thus, when the fiber reinforced coupler is stressed by internal fluid pressures in the direction of the fiber placement, the applied load is withstood primarily by the included fibers and the coupler strength in resisting such stress is at a maximum value. Conversely, when the composite member is stressed in a perpendicular direction to the fiber direction, the applied force must necessarily be resisted primarily by the polymer matrix so that the coupler strength is at minimum. The relative amounts of the individual stresses being applied to the fiber reinforced coupler must also necessarily be considered for proper fiber placement direction. For an externally unconstrained installation of said previously disclosed pipe couplings, such as encountered with above ground pipe installations, the applied loads can be examined by treating the joined pipe lengths as a pressure vessel. From such analysis it was found that the stress applied to the pipe wall in the hoop direction is twice an amount as the applied stress in the pipe's axial direction. Employing well recognized shell theory calculation, it was further found that a fiber-angle of 55 degrees was needed to balance these applied loads assuming 90 degrees to be in the pipe hoop direction and 0 degrees to be aligned in the direction of the pipe longitudinal axis. For constrained pipe installations, however, such as in-ground or having the pipe ends being held there, there can only be need for resisting hoop stress. Accordingly, fiber placement at or near a 90 degree angle with respect to the longitudinal pipe axis was dictated while further recognizing that some angle less than 90 degrees may only be achievable with the fiber winding in the customary manner. The entire contents of said referenced co-pending application are hereby specifically incorporated into the present application.
It can readily be appreciated that thermoplastic storage vessels undergo similar internal stress when being utilized. Accordingly, the effectiveness of fiber reinforcement for thermoplastic storage vessels will also depend to a considerable degree upon the same factors previously considered with respect to said reinforced thermoplastic couplings. For example, a thermoplastic storage vessel having a cylindrical configuration can generally have the fiber wraps applied in a hoop direction for maximum reinforcement whereas a spherical storage vessel will understandably have the fiber placement angle varied in different spatial directions. It has now been found, however, that thermal bonding the reinforcement fibers to the outer surface of the thermoplastic storage vessel in the same manner previously employed for reinforcement of said thermoplastic pipe couplings produces inferior results. Specifically, the previously employed bonding method provided sufficient thermal expansion of the thermoplastic inner coupling member when being carried out that an effective thermal bonding with the applied fiber reinforcement took place. This does not reliably occur for various shaped thermoplastic storage vessels having a lesser wall thickness. It thereby becomes necessary for said relatively thin wall storage vessels to adopt an improved thermal bonding procedure for the fiber reinforcement to have the desired effectiveness.
It is an important object of the present invention, therefore, to provide a novel method to reinforce thin wall thermoplastic storage vessels with one or more wraps of applied continuous fiber.
It is still another object of the present invention to provide a novel method to secure the applied fibers to the outer surface of a thin wall thermoplastic storage vessel so as to better resist internal stress when the storage vessel is in use and prevent delamination when pressure is released.
Still another object of the present invention is to provide a novel method for reinforcement of a thin wall thermoplastic storage vessel which includes a plurality of continuous juxtapositioned fibers being reliably secured to the outer surface of said storage vessel so as to be aligned in a predetermined spatial direction resisting applied internal stress during vessel use.
These and still further objects of the present invention will become more apparent upon considering the following more detailed description of the present invention.